1. Field of the Invention
This invention relates generally to a stencil printer, and more particularly to a polychromatic stencil printer which disposes a plurality of printing drums each having a stencil paper wound around the outer peripheral surface thereof, around a single paper pinch drum so that a polychromatic image such as a color image can be formed on printing paper held on the outer peripheral surface of the paper pinch drum.
2. Description of the Related Art
A conventional polychromatic stencil printer is shown in Japanese Unexamined Patent Publication No. 4(1992)-105984 by way of example. In such a stencil printer, a plurality of printing drums are disposed around a paper pinch drum. The paper pinch drum is formed with a cutout along the generating line of the outer peripheral surface. Each printing drum has a stencil paper anchor, which anchors one end of a stencil paper wound around the outer peripheral surface of the drum, on a portion along the generating line of the outer peripheral surface and includes an internal press roller (an ink-containing roller) in the interior. In rotating the paper pinch drum and the printing drum in opposite directions in synchronization with each other, with the cutout and the stencil paper anchor opposed to each other, ink is supplied to the stencil paper via the internal press roller, whereby printing is performed on the printing paper held on the paper pinch drum.
In the conventional polychromatic stencil printer of the above kind, incidentally, polychromatic printing can be performed on one side of the printing paper by single paper conveyance. However, when duplex polychromatic printing is performed, the printing paper printed on one side must be turned over and reprinted.
In view of the foregoing observations and description, the primary object of the present invention is to provide a stencil printer which is capable of easily performing duplex polychromatic printing by single paper conveyance similarly to single-side polychromatic printing.
To achieve this end, the stencil printer according to the present invention is provided with position control means. The position control means independently controls movement of the internal press roller of each printing drum between an operative position which performs printing on printing paper and an inoperative position which performs no printing. In this way, it is easy to perform single-side monochromatic printing with all the printing drums. It is also easy to perform single-side printing with at least one printing drum selected from a plurality of printing drums. Furthermore, performing duplex printing is easy.
In a preferred form of the present invention, the stencil printer further includes reference position detection means which detects a reference position in the direction of rotation of the paper pinch drum or a plurality of printing drums and rotational position detection means which detects a position of rotation from the reference position of the paper pinch drum or the plurality of printing drums, based on information detected by the reference position detection means. The position control means controls the internal press roller, based on information from the rotational position detection means. In this way, the position control means can move the internal press roller at correct timing.
In another preferred form of the present invention, the position control means moves the internal press roller to the operative position when the position of rotation of the paper pinch drum or the plurality of printing drums, detected by the rotational position detection means, coincides with a predetermined operation start point for the internal press roller.
In still another preferred form of the present invention, the position control means moves the internal press roller to the inoperative position when the position of rotation of the paper pinch drum or the plurality of printing drums, detected by the rotational position detection means, coincides with a predetermined operation end point for the internal press roller.
In the case where printing is performed on one side of the printing paper, mirror-image stencil papers which can print a non-reverse image on the obverse side of the printing paper are wound around the plurality of printing drums, and the position control means moves the internal press rollers of the plurality of mirror image printing drums to the operative positions in order from the internal press roller of the mirror image printing drum on an upstream side of a conveying path of the printing paper.
In the case where printing is performed on both sides of the printing paper, a mirror image stencil paper which can print a non-reverse image on the obverse side of the printing paper is wound around at least one of the plurality of printing drums, and a non-reverse image stencil paper which can print a mirror image on the outer peripheral surface of the paper pinch drum is wound around at least one of the remaining printing drums. The position control means moves only the internal press roller of the non-reverse image printing drum to the operative position and prints the mirror image on the outer peripheral surface of the paper pinch drum, without winding the printing paper around the paper pinch drum, and then moves the internal press roller of the non-reverse image printing drum to the inoperative position and the internal press roller of the mirror image printing drum to the operative position, with the printing paper wound around the paper pinch drum, and prints the non-reverse image on the obverse side of the printing paper and transfers the mirror image printed on the outer peripheral surface of the paper pinch drum to the reverse side of the printing paper as a non-reverse image.
In this case, the printing drum of the plurality of printing drums on the most downstream side of the conveying path of the printing paper can be employed as a press printing drum winding a non-perforated stencil paper. In this way, a stable transfer of an image to the reverse side of printing paper can be performed without providing an additional pressure roller.
In a further preferred form of the present invention, the paper pinch drum is directly driven to rotate by a drive source via a first rotation transmission system and the plurality of printing drums are driven to rotate by the paper pinch drum via a second rotation transmission system. Because a plurality of printing drums can be driven under the same condition, a phase difference between the printing drums can be prevented.
Furthermore, it is preferable that guide rollers, which press the printing paper on the paper pinch drum by contacting the paper pinch drum being driven to rotate and are driven by the paper pinch drum, be disposed between the plurality of printing drums. When only a specific printing drum of a plurality of printing drums is employed in printing, the rear end of printing paper can be prevented from being stained with ink in the non-printing drums.